This invention relates to a fuel injector for use in supplying fuel to a combustion space of an internal combustion engine. Such a fuel injector may be suitable for use in, for example, a common rail type fuel system and for control by an electronic control arrangement.
It is often a requirement to be able to vary the rate at which fuel is delivered by the injector. Commonly, the fuel injection rate is dependent upon the distance by which a valve needle is lifted away from its seating, movement of the valve needle within a bore provided in a nozzle body being controlled by means of a piezoelectric actuator. The piezoelectric actuator is operable to control the position occupied by a control piston, the piston being moveable to control the fuel pressure within a control chamber defined, in part, by a surface associated with the valve needle of the injector to control movement of the valve needle away from its seating. When the valve needle is lifted away from its seating into a first fuel injecting position, a set of upper outlet openings are exposed and fuel is delivered therefrom. When the valve needle is lifted away from its seating into a second fuel injecting position, a set of lower outlet openings are also exposed, fuel thereby being delivered through both sets of outlet openings to increase the fuel injection rate.
A problem with two-stage fuel injectors of the aforementioned type is that, when fuel delivery is to be terminated, the sudden closure of the lower set of outlet openings results in a rapid increase in fuel pressure in the tip of the nozzle body. If the pressure increase coincides with peaks in the pressure waves in the supply drillings, this can lead to an adversely slow closure of the outlet opening. Hence, fuel injection is terminated relatively slowly which can lead to a poor fuel spray characteristic and poor injector performance. Secondly, there is a tendency for the valve needle to oscillate between the first and second fuel injecting positions as there is no mechanical stop to limit the extent of movement of the valve needle away from its seating. This can lead to poor controllability. In addition, it is necessary for movement of the valve needle to the first and second fuel injecting positions to be of a relatively large magnitude to ensure that the valve needle remains in the second fuel injecting position for a period of time which is sufficient to permit an adequate quantity of fuel to be delivered, and also to ensure there is a dead band, when fuel injection takes place through the upper openings only, prior to opening of the lower set of outlet openings.
It is an object of the present invention to provide a fuel injector which alleviates the aforementioned problems of the prior art.
According to a first aspect of the present invention, there is provided a fuel injector comprising a valve needle which is slidable within a bore and engageable with a seating to control fuel delivery through first and second outlet openings, the valve needle being moveable between a closed position and first and second fuel injecting positions, a fuel supply passage for supplying fuel under pressure to the bore, a control chamber which is arranged to receive fuel from the fuel passage, in use, the valve needle being acted upon in use by fuel pressure within the control chamber, an actuator arrangement for controlling fuel pressure within the control chamber, and damping means for damping movement of the valve needle away from the seating into the first or second fuel injecting position.
By damping movement of the valve needle as it moves away from the seating into either the first or second fuel injecting position, the problem of valve needle oscillation can be obviated or mitigated. In addition, by damping movement of the valve needle away from the seating, the first and second outlet openings can be arranged such that the valve needle need only be moved away from the seating by a relatively small amount into the first and/or second fuel injecting position.
Conveniently, movement of the valve needle away from the seating into the first fuel injecting position causes fuel to be delivered through the first outlet opening and movement of the valve needle away from the seating into the second fuel injecting position causes fuel to be delivered through the first and second outlet openings. Thus, movement of the valve needle into the second fuel injecting position causes fuel to be delivered at an increased rate.
Preferably, the control chamber is divided into a first part associated with the valve needle and a second part associated with the actuator, and the damping means include a flow restrictor which serves to restrict flow of fuel, in use, from the first part of the control chamber to the second part of the control chamber.
The control chamber and flow restrictor may be arranged such that, in use, flow of fuel takes place through the flow restrictor from the second part of the control chamber to the first part of the control chamber. In this embodiment, the flow restrictor is preferably arranged such that there is a relatively lower restriction to fuel flow in this direction.
The flow restrictor may be defined by a drilling provided in a housing part. Conveniently, the flow restrictor may take the form of a restricted flow passage of stepped form or may take the form of a venturi-type flow passage.
The injector may preferably comprise an inner valve needle which is movable within the valve needle upon movement of the valve needle away from the seating beyond a predetermined amount, the inner valve needle being engageable with a further seating to control fuel flow through the second outlet opening.
The supply passage may include an additional restricted flow passage which serves to limit the amplitude of pressure waves within the supply passage. In this way, movement of the valve needle against the seating to terminate fuel injection is not caused to be slowed due to the transmission of large amplitude pressure waves through the supply passage. Preferably, the restricted passage is of a dimension which does not give rise to a substantial decrease in fuel pressure between the inlet and outlet ends of the restricted passage.
According to as second aspect of the invention, there is provided a fuel injector comprising a valve needle which is slidable within a bore and engageable with a seating to control fuel delivery through an outlet opening, a fuel supply passage for supplying fuel under pressure to the bore, a control chamber which is arranged to receive fuel from the fuel passage, in use, the valve needle being acted upon in use by fuel pressure within the control chamber, an actuator arrangement for controlling fuel pressure within the control chamber, wherein the supply passage includes an additional restricted flow passage which serves to limit the amplitude of pressure waves within the supply passage.